In the Graphic Arts Industry, low molecular weight (M.sub.n &lt;20,000), alkali-soluble, high T.sub.g (&gt;50.degree. C.), carboxylated styrene polymers have been used as pigment dispersants, overprint varnishes, and for preparing resin fortified latexes for water based inks and coatings. These polymers are very polar due to their carboxyl content and are, therefore, synthesized in a polar solvent. Conventional solvents employed in their production include low boiling alcohols such as ethyl or isopropyl alcohol, ketones such as methyl isobutyl ketone and high boiling ethylene glycol based solvents such as diethylene glycol monoethyl ether (DEGEE) or diethylene monoethyl ether acetate (DEGEEA). The ethylene glycol based solvents have been preferred because (a) they are water-soluble and since the resins are used as their ammoniacal solutions residues of non-water-soluble solvents cause the solution to be hazy, (b) the solvents afford an ability to polymerize at a high temperature (the glycol solvents are used at 150.degree. to 250.degree. C.) which leads to greater chain transfer and lower molecular weight, (c) polymerization at a high temperature permits the use of lower cost, more thermally stable, safer-to-use and store organic peroxides such as di-t-butyl peroxide, t-butyl perbenzoate, and 2,5-di-t-butylperoxy-2,5-dimethylhexane, and (d) the presence of the pendant glycol groups incorporated into the polymer by esterification (of the solvent) with the carboxylic acid groups or by chain transfer enhance the performance of the resin. (Ink World, 2 (6), 1996, p 32-37)
Although historically, the industry choice has been the ethylene glycol based solvents, such ethylene glycol based solvents have been designated hazardous air pollutants (HAPs). Several have been determined to be teratogenic. Threshold reporting levels for accidental spills are as low as one pound. Therefore, the printing ink industry is demanding resins which are completely free of ethylene glycol based solvents because it is virtually impossible to completely remove all traces of these ethylene glycol based solvents from the solution polymerized resin.
Representative patents exemplifying this technology are as follows:
Canadian Patent 814,528 discloses the production of alkali soluble resins which are described as having a number average molecular weight of from about 2,000 to 3,600 and a high acid number which, in alkali solubilized form, are useful as emulsifiers and leveling agents. Preferred polymers are based upon styrene and (meth)acrylic acid which are polymerized in the presence of a chain transfer agent or chain transfer solvent. Chain-transfer reaction solvents employed in the process include methyl isobutyl ketone, xylene, methyl isobutyl carbinol and the like.
U.S. Pat. No. 4,151,341 discloses low molecular weight interpolymers containing lower alkyl capped poly(oxyethylene) ether groups which are employed as aqueous dispersing agents for particulate matter, e.g., difficult-to-disperse finely divided inorganic solids such as calcium carbonate. The polymer is based upon a hydrophobic monomer such as styrene, a copolymerizable ethylenically unsaturated carboxylic acid, such as acrylic or methacrylic acid, and an alkenyl benzyl ether, where the ether is the alkyl capped poly(oxyethylene) ether. Polymers based upon styrene, maleic anhydride and a vinyl benzyl ether having pendent methyl capped ethoxylated nonylphenol groups are disclosed. Also disclosed are low molecular weight styrene, maleic anhydride copolymers wherein a portion of the maleic anhydride repeating units are half esterified with a polyethylene glycol monomethyl ether.
U.S. Pat. No. 4,025,484 discloses dispersions of finely divided particulate solids in an organic medium stabilized by addition of a polymeric dispersant which is based upon a copolymer of styrene, maleic anhydride and a vinyl benzyl ether of an ethoxylated nonylphenol. Alkyl half esters of maleic and fumaric acids as well as alkyl half amides of maleic and fumaric acids, acrylic and methacrylic acids, are suggested as suitable for producing the polymeric dispersant.
U.S. Pat. No. 3,392,155 discloses low molecular weight styrene/maleic anhydride copolymers esterified with monohydric polyether alcohols. The copolymers are based upon styrene and maleic compounds of very low molecular weight, e.g., those having a solution viscosity up to only 2.0 centistokes (10 g copolymer/100 ml acetone). Preferred copolymers are terminated by a chain terminating solvent, which is an alkyl-substituted aromatic organic compound such as an isopropyl-substituted benzene or ketone. Monohydroxypolyoxyalkylene glycols used as the esterifying ether alcohols have molecular weights which range from about 120 to 10,000, and examples include diethylene glycol monobutyl ether and ethylene oxide condensation products of primary, secondary and tertiary alcohols. Example 8 shows production of a copolymer of styrene and the maleic acid half ester of polyethylene glycol monomethyl ether having a molecular weight of about 350.
U.S. Pat. No. 5,130,369 discloses the production of functionalized polymeric compositions by polymerizing an ethylenically unsaturated functional monomer in a solvent including a reactive compound. An example of a functional/reactive group is a carboxylic acid/primary alcohol group. Ethylenically unsaturated monomers having carboxyl functionality include acrylic and methacrylic acid and .alpha.,.beta.-unsaturated dicarboxylic acids and their anhydrides, such as itaconic, maleic and fumaric acid, and their anhydrides. Examples of hydroxy functional surface active compounds include alkyl aryloxypoly(ethyleneoxy)ethanol and block copolymers of polyethylene oxide and polypropylene oxide, e.g., such as those sold under the trademark Pluronic, and the like. Exemplary copolymers are based upon styrene, acrylic acid and a C.sub.12 alkoxy(ethylene oxide)ethanol, or styrene, acrylic acid and a polyethylene glycol.